The digestion of 125I-asialo-fetuin within lysosomes will be inhibited in vivo and in the perfused liver. This inhibition will be accomplished by directing both leupeptin (an inhibitor of the thiol requiring catepsins B, H and L) and pepstatin (an inhibitor of cathepsin D) into the hepatocyte lysosomes. Leupeptin has already been shown to be effective in these two biological systems, however pepstatin does not appear to enter cells readily. Pepstatin will therefore be chemically bonded to asialo-fetuin by formation of peptide linkages between its carboxyl function and the epsilon-amino groups of lysines in the glycoprotein. The pepstatin then can be carried into the hepatocyte lysosomes during the uptake of this glycoprotein by the liver. Either the inhibitor-glycoprotein complex itself will stop the action of cathepsin D, or the lysosomal proteinases will degrade this conjugate to yield a small pepstatin-containing fragment that is again an effective inhibitor of cathepsin D. Additional studies will be done to characterize the carbohydrate residues present on the exterior surface or purified lysosomes (isolated from the livers of rats treated with Triton WR-1339). The lysosomes will be fixed with glutaraldehyde to stabilize their membrane and then reacted with plant lectins. The agglutinability of the lysosomes by conconavalin A, wheat germ agglutinin and ricin I will be measured. Specific binding to the lysosomal organelles by these same lectins (labeled with 125I) will be analyzed as well.